1. Technical Field
The invention relates generally to ion implantation and more particularly to a method, system, and apparatus for improving doping uniformity in a high-tilt ion implantation.
2. Related Art
Ion beam implanters are widely used in the production of semiconductor wafers. An ion beam implanter generates a beam of positively-charged ions which, when applied to a surface of a semiconductor wafer, are implanted or “doped” onto the wafer surface. For example, FIG. 1 shows an orientation 100 of a wafer 110 in three dimensions X, Y, and Z. A planar surface of wafer 110 is oriented parallel to the X-axis and Y-axis, but perpendicular to the Z-axis. As such, an ion beam (not shown) delivered parallel to the Z-axis will strike the planar surface of wafer 110 at a 90 degree angle.
It may be desirable, however, to angle wafer 110 relative to the ion beam, such that the ion beam strikes the planar surface of wafer 110 at an angle other than 90 degrees. This may be done for any number of reasons, including, for example, in an attempt to prevent a channeling effect or to improve ion implantation on sidewalls of trench structures on the planar surface of the wafer. For example, FIG. 2 shows an orientation 200 of wafer 210 in three dimensions X, Y, and Z, wherein the planar surface of wafer 210 has been rotated about the X-axis such that an angle 212 (i.e., tilt angle θA) is formed between wafer 210 and the Y-axis. Such an orientation is commonly employed, as noted above, to prevent a channeling effect or improve sidewall ion implantation.
However, orientations such as that of FIG. 2 are not without drawbacks. For example, the orientation 200 of FIG. 2 is non-isocentric, meaning that the position along the Z-axis at which an ion beam strikes wafer 210 varies relative to the Y-axis. The degree of variance generally increases as angle 212 increases and as the size of wafer 210 increases. Such variance may be extreme in high-tilt ion implantations, where angle 212 is often between about 10 degrees and about 60 degrees. Such differences may result in differences in ion beam currents and/or current densities, resulting in variances in dopant distribution along the surface of wafer 210.
Such variance may similarly be affected by structures on the surface of wafer 210. For example, referring to FIG. 3, a side view of a high-tilt orientation 300 of wafer 310 is shown. Wafer 310 has been rotated about the X-axis (into page) to form angle 312 between wafer 310 and the Y-axis. Wafer 310 includes a plurality of trench structures 314 on its surface. Trench structures 314 may be any known or later developed structure or feature forming sidewalls relative to a planar surface of wafer 310, including, for example, a trench, a channel, and a via.
Still referring to FIG. 3, an ion beam 320 directed parallel to the Z-axis strikes wafer 310 at different points along the Z-axis, due to the angle 312 of wafer 310 relative to the Y-axis. In addition, although ion beam 320 may be directed at equally spaced transmission intervals D1-5 relative to the Y-axis, due to trench structures 314, ion beam 320 will strike wafer 310 at varying incident intervals d1-5 relative to the Z-axis. Some incident intervals, such as d1 and d5, may be less than their corresponding transmission intervals (i.e., D1 and D5). Other incident intervals, such as d2, may be greater than their corresponding transmission intervals (i.e., D2). This situation further exacerbates the differences in ion beam currents and/or current densities along the Y-axis of the wafer caused by angle 312. As a result, dopant distribution may vary along the Y-axis of the wafer, the distribution being higher or lower at the wafer's edges relative to its center.
Others have described methods aimed at minimizing this variance in dopant distribution. For example, U.S. Pat. No. 6,313,474 to Iwasawa et al. describes a method comprising measuring a current density distribution at each of two points along a Z-axis and interpolating and/or extrapolating a current density distribution along the entire Z-axis. However, such a method provides only an approximation of the actual current density distribution along the wafer surface.
Accordingly, there is a need in the art for a method, system, and apparatus for improving doping uniformity during a high-tilt ion implantation.